Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. When the vehicle is first started, the PCM controls fuel delivery in open loop, ignoring the heated oxygen sensor (HO2S) 1 signal and calculating air/fuel ratio based on inputs from the engine coolant temperature, throttle position, and mass air flow sensors. The PCM will begin using the HO2S 1 signal for controlling fuel delivery (Closed Loop) when the following conditions have been met:
| • | The HO2S 1 signal is varying, indicating that the sensor is hot enough to operate properly. |
| • | The engine coolant temperature sensor indicates coolant temperature above 20°C (68°F). |
| • | Time since startup is at least 10 seconds for a warm engine or 50 seconds for a cold engine. |
Conditions for Running the DTC
| • | No IAT or ECT sensor DTC(s) present |
| • | Intake air temperature is greater than -40°C (-40°F). |
| • | Engine coolant temperature is greater than -40°C (-40°F). |
Conditions for Setting the DTC
The length of time for the engine coolant to reach a stabilized temperature of 20°C (68°F) takes longer than it should. The amount of time ranges between 2 and 5 minutes depending on engine coolant temperature at startup and the amount of air cycled by the engine since startup.
Action Taken When the DTC Sets
| • | The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool Clear Info function or by disconnecting the PCM battery feed. |
Diagnostic Aids
The DTC P0125 diagnostic test will not run if the engine is started while the coolant temperature is less than 0°C (32°F) or greater than 21°C (70°F). If it is not possible to operate the vehicle under the appropriate conditions, check the following items:
| • | Cooling system coolant level. |
| Refer to Cooling and Radiator . |
| • | Thermostat operation. |
| Refer to Cooling and Radiator . |
| • | Cooling fans operation. |
| Refer to Electric Cooling Fan Diagnosis. |
| • | Check for high resistance in wiring related to the ECT sensor. Also, check for poor connections at ECT sensor and PCM. Refer to Troubleshooting Procedures in Electrical Diagnosis. |
| • | DTC P0125 set indicates a skewed ECT sensor. Comparing the engine coolant temperature displayed on a scan tool with actual coolant temperature measured with a thermometer may isolate this condition. If the displayed engine coolant temperature is not close to the actual coolant temperature, replace the ECT sensor. |
| Refer to Temperature vs Resistance . |
| • | Poor connection at PCM. |
| Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness. |
| Inspect the wiring harness for damage. If the harness appears to be OK, observe the ECT display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the display will indicate the location of the malfunction. |
If DTC P0125 cannot be duplicated, the information included in the Fail Records data can be useful in determining vehicle mileage since the DTC was last set.
Test Description
An example test description follows:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||
2 | Are any of the following ECT sensor DTCs set? DTC P0117 Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage DTC P0118 Engine Coolant Temperature (ECT) Sensor Circuit High Voltage DTC P1114 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage DTC P1115 Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage | -- | Go to the other DTC first | |
3 | While allowing the engine to completely cool, turn the ignition switch ON (engine OFF) and observe the cooling fans operation. Are the cooling fans OFF? | -- | ||
4 |
Does the engine coolant temperature increase to above the specified value within 2 minutes? | 21°C (70°F) | Refer to Diagnostic Aids | |
5 | Check the cooling system coolant level. Refer to Engine Cooling. Is the coolant level OK? | -- | ||
6 | Check for proper operation of the thermostat . Refer to Engine Cooling. Is the thermostat operating correctly? | -- | ||
Compare Engine Coolant Temperature displayed on the scan tool to the actual coolant temperature measured with a thermometer. Is the scan tool Engine Coolant Temperature indication within the specified value of the measured temperature? | ± 3°C (± 5°F) | |||
8 |
Was a problem found? | -- | ||
9 | Refer to Cooling and Radiator for cooling system diagnosis and repair condition as necessary. Is action complete? | -- | -- | |
10 | Replace the loose terminal(s) or repair malfunctioning wiring as necessary. Refer to Repair Procedures in Wiring Repairs . Is action complete? | -- | -- | |
11 | Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement Is action complete? | -- | -- | |
12 |
Does Eng. Cool. Temp. increase to above the specified value within 2 minutes? | 21°C (70°F) | System OK |
